The present invention relates to vibratory conveying equipment for moving bulk material, and, more particularly, to a differential motion conveyor drive.
Vibratory conveyors are well known is the art and are commonly used for moving bulk materials. There are two primary types of vibratory conveyors. In a first type, a force is imparted to the material-carrying trough at an angle relative to horizontal plane defined by the trough so that the material moves with the trough along this angle. Once the force is reversed, the trough moves in the reverse direction, allowing the material to fall to the trough in a more forward position. Typically, an eccentric shaft drive system imparts the requisite force to the trough, which is mounted to a stationary support through a plurality of elastic members such as springs. The eccentric shaft drive system comprises one or more rotating shafts and associated drive springs so as to impart the cyclically acting resultant force to the vibratory trough. In simple terms, the conveyed material is essentially xe2x80x9cbouncedxe2x80x9d along the trough from an inlet end to a discharge end. Thus, through much of its travel, the conveyed material is airborne, and the material actually contacts the trough only long enough to be re-launched into the air in the direction of the discharge end of the trough.
For various reasons, it may not be desirable to use the aforementioned vibratory conveyor as the xe2x80x9cbouncingxe2x80x9d action is not without its disadvantages. Quite obviously, the movement of the conveyor trough in both horizontal and vertical directions makes it extremely difficult for a worker stationed along the conveyor trough to perform any work on the conveyed material. Furthermore, the vertical motion of the conveyor trough may cause light particulates to be launched into the air, creating an extremely dusty and potentially hazardous working environment. Finally, the repeated impact of the material against the conveyor trough as it launches and catches the material creates a noisy working environment, and the repeated impact can also cause damage to the conveyor trough and the product conveyed over time.
One solution to this problem has been the development of a second type of vibratory conveyor in which the conveying action is achieved through the application of a purely horizontal force to the conveyor trough, with the supports for the trough permitting movement of the trough primarily horizontally. In other words, any vertical movement of the trough is minimal.
In such a vibratory conveyor, commonly referred to as a xe2x80x9cslow advance, fast return,xe2x80x9d xe2x80x9cdifferential motionxe2x80x9d or xe2x80x9chorizontal motionxe2x80x9d conveyor, the drive or vibration-generating means is arranged such that the maximum horizontal vibratory forces applied to the trough in the direction of conveyance are less than the static friction force acting between the trough and the conveyed material. As such, adherence is maintained between the conveyed material and the trough, and the material is conveyed forward; however, the horizontal force applied to the trough opposite to the direction of conveyance is such that the static frictional force is exceeded, and the trough recoils without returning the material to its original position. In other words, the conveyor trough moves forward relatively slowly, keeping acceleration down so the conveyed material will not slip, but the conveyor trough moves backward and returns at a much higher speed and acceleration, overcoming frictional forces in that the material slides relative to the conveyor trough. As such, a xe2x80x9cslow advance, fast returnxe2x80x9d or xe2x80x9cshufflingxe2x80x9d motion is achieved.
U.S. Pat. No. 5,131,525 (xe2x80x9cthe ""525 Patent), which is assigned to the General Kinematics Cooperation of Barrington, Ill., describes such an exemplary differential motion conveyor. For this teaching, the ""525 Patent is incorporated herein by this reference. Furthermore, as recognized in the ""525 Patent, the differential motion conveyor can be used to convey powdery or dusty materials without creating an extremely dusty and potentially hazardous working environment, and also allows for the visual and/or manual inspection and treatment of materials by workers positioned along side the conveyor trough. As stated in the ""525 Patent, other potential advantages of a differential motion conveyor include: the ability to position the drive or vibration-generating means anywhere along the conveyor trough; and the ability to secure the drive or vibration-generating means directly to the trough as the forces generated by the drive are absorbed by the reactive motion of the trough, thereby reducing the reaction forces which must otherwise be absorbed.
For further information about prior art differential motion conveyors and the development of such conveyors, reference is also made to U.S. Pat. No. 5,850,906 (xe2x80x9cthe ""906 Patent), which is assigned to FMC Corporation of Chicago, Ill. The ""906 Patent is incorporated herein by this reference.
In any event, most prior art differential motion conveyors employ rotating weight, brute-force drives that include four drive shafts. The relative position of the drive shafts is held constant through the use of gears or gear belts. Referring now to FIGS. 2A-2D, two of the drive shafts 10A, 10B function as a pair, carrying equivalent eccentric weights 12A, 12B and counter-rotating in the directions indicated by arrows 14A and 14B at a chosen operating speed. The two remaining drive shafts 20A, 20B similarly function as a pair, carrying equivalent eccentric weights 22A, 22B (larger than those carried by the first pair of drive shafts) and also counter-rotating in the directions indicated by arrows 24A and 24B, but at an operating speed half that of the first pair of drive shafts 10A, 10B. Thus, the rotating forces represented by the eccentric weights are always additive in the plane of the conveyor trough (i.e., horizontal), but always cancel in any other direction relative to the plane of the conveyor trough (i.e., vertical).
FIG. 3A depicts the net acceleration output from such a four-shaft prior art drive as a function of time. Specifically, the net acceleration from the drive is the sum of two sinusoidal accelerations of different amplitude and frequency. As indicated in FIG. 3A, at t=0 s, when the position of the eccentric weights is that depicted in FIG. 2A, a substantial negative acceleration is generated by the conveyor drive, imparting a force on the conveyor trough that results in the xe2x80x9cfast return,xe2x80x9d the backward movement of the conveyor trough that overcomes the frictional forces between the conveyed material and the conveyor trough.
As the eccentric weights rotate into the position depicted in FIG. 2B (t=0.043 s), a positive acceleration and resultant force (albeit with a magnitude less than that of the negative xe2x80x9cfast returnxe2x80x9d force) is generated to convey the material forward in the conveyor trough. This positive acceleration and force, and the resulting forward movement, continues as the eccentric weights rotate into the positions depicted in FIGS. 2C (t=0.086 s) and 2D (t=0. 129 s).
Finally, as the eccentric weights return to the position depicted in FIG. 2A, another rapid backward movement is generated, again overcoming the frictional forces between the conveyed material and the conveyor trough.
As should be clear from a review of FIG. 3A, to maximize the net conveying speed, it is desirable to minimize the time interval in which a negative acceleration and resultant force is generated and acts on the conveyor trough. At the same time, it is desirable to maximize the time interval in which a positive acceleration and resultant force is generated, an interval generally bounded by the positions of the eccentric weights depicted in FIGS. 2B and 2D.
FIG. 3B depicts the velocity of the of the conveying motion as a function of time. As shown, the velocity oscillates over time between a minimum at approximately the position of the eccentric weights as depicted in FIG. 2B to a maximum at approximately the position of the eccentric weights as depicted in FIG. 2D. As will be described in further detail in the description of the present invention, the ideal velocity profile would indicate a constant positive acceleration (i.e., forward movement) with a virtually instantaneous negative acceleration (i.e., backward movement).
Finally, FIG. 3C depicts the motion (i.e., displacement) of the conveyor trough as a function of time.
Prior art differential motion conveyors employing rotating weight, brute-force drives with four drive shafts have been demonstrated to operate well up to conveying speeds of 30-40 feet per minute (fpm). The limiting factor, however, for such a differential motion conveyor is the maximum G-differential that can be obtained using the four-shaft arrangement. The G-differential is the ratio between the G-loads at the two extremes of the operating stroke (i.e., one complete rotation of the first pair of drive shafts), i.e., the ratio of the minimum acceleration to the maximum acceleration imparted to the conveyor trough:                               G          ⁢                      -                    ⁢          differential                =                              -                          xe2x80x83                        ⁢                                          min                ⁡                                  (                                      G                    LOAD                                    )                                                            max                ⁢                                  xe2x80x83                                ⁢                                  (                                      G                    LOAD                                    )                                                              =                      -                          xe2x80x83                        ⁢                                          min                ⁢                                  xe2x80x83                                ⁢                                  (                  acceleration                  )                                                            max                ⁢                                  xe2x80x83                                ⁢                                  (                  acceleration                  )                                                                                        (        1        )            
For a four-shaft drive, the theoretical maximum G-differential is 2.0.
It is therefore a paramount object of the present invention to provide a differential motion conveyor drive that allows for higher conveying speeds, speeds that are not possible due to the limitations inherent in the prior art.
This and other objects and advantages of the present invention will become apparent upon a review of the following description and appended claims.
The present invention is a differential motion conveyor drive employing at least three pairs of drive shafts carrying eccentric weights that are selected to correspond to the terms of a Fourier series in order to achieve optimal conveying speed and efficiency. To approximate the Fourier series terms, in one preferred embodiment, each of the first pair of shafts carries a weight WR and rotates at a predetermined operating speed xcfx89. Each of the second pair of shafts carries a weight (0.50)WR and rotates at an operating speed 2xcfx89, and each of the third pair of shafts carries a weight (0.33)WR and rotates at an operating speed 3xcfx89. Since a Fourier series contains an infinite number of sinusoidal terms, however, the actual weight distribution on the drive shaft pairs may be further adjusted to account for the xe2x80x9cmissingxe2x80x9d terms of the Fourier series. An appropriate weight distribution results in a differential motion conveyor drive that consistently produces a higher conveying speed compared to that produced by a prior art drive throughout a range of operating speeds.